Vitrification of waste material is becoming the preferred route for the treatment and remediation of non-combustible wastes. Vitrification reduces the amount of hazardous waste generated from solid waste combustion furnaces. A wide variety of non-combustible waste, including (but not limited to) contaminated soils, process waste and post combustion residuals (ashes) containing inorganic as well as organic compounds can be effectively treated and incorporated into a siliceous (glass) mass without significant deterioration of the ability of the glass to lock up and retain the inorganic contaminants, in effect, permanently. Permanent retention of contaminants by vitrification refers to retention in which any leakage of contaminants is so slow that no detectable adverse environmental effects are produced.
Ex situ vitrification, as opposed to in situ vitrification, has the advantage of increased control before and during processing and ultimately increased control of the consistency of the vitrified product. A number of vitrification processes have been utilized, and they include those described in U.S. Pat. Nos. 5,873,675; 5,603,684; and 5,662,579. A general summary is provided in the EPA Handbook entitled xe2x80x9cVitrification Technologies for Treatment of Hazardous and Radioactive Wastexe2x80x9d.
Various problems are encountered when using the vitrification processes currently utilized in the art. The greatest problem is the escape of hazardous, volatile inorganic and organic materials during pre-treatment of waste material, as well as during vitrification. Particularly troublesome is the escape of hazardous metal compounds such as mercury, arsenic, antimony, zinc, cadmium, chromium, and lead compounds.
Some waste materials are treated prior to undergoing vitrification. Crushers and grinders are typically used to increase the surface area of the waste material, thereby permitting the use of more consistent, and lower, temperatures during the melt process. However, this method, which involves various residence times, not only requires high amounts of energy, but has the undesired side effect of volatilizing hazardous compounds due to increased temperatures applied during pretreatment. For example, when rotary dryers are utilized to pretreat or dry the waste material, a dried, agglomerated product is obtained, reducing the surface area of the waste material. The temperatures therefore required to dry the waste material using rotary dryers are basically too high, and consequently the adsorbent material used to filter dust particles from the exhaust gases is rendered ineffective. Rendering the adsorbent material ineffective allows volatile hazardous materials to escape, and therefore additional techniques must be used to trap, treat, and dispose of the released hazardous materials.
During exposed conventional glass tank surface vitrification, volatile compounds can escape from the melt. This is believed to be due in part to the presence of water and in part due to residual carbon from degraded, waste-associated, organic materials. Upon melting of the waste material residual, carbon causes various toxic, inorganic metal oxides to become reduced to their zero valent state, which are generally more volatile. The combination of heated, entrapped water vapor, carbon-water shift reaction by-products (carbon monoxide and hydrogen) and oxidized combustion gases (CO2) cause the more volatile toxic metals to escape the melt and subsequently form gaseous element fumes and/or compounds. Not only do these materials exit the melted waste material in gaseous form, but they require additional steps to recover and dispose of the hazardous materials. Such hazardous materials may contain metals or metal compounds which are environmentally dangerous. The disposal of such materials is therefore closely regulated by governmental agencies.
It is an object of this invention to provide a process and apparatus for treating waste materials containing volatile organic and inorganic compounds, wherein the waste material is melted in a cold top (cap) furnace and ultimately converted into a benign, innocuous, disposable vitrified mass or product, and wherein, in the course of melting and vitrification, leakage of volatile metals and compounds into the environment is greatly reduced.
In accordance with the invention, the-waste is pretreated to decrease its water content without significant volatilization of low volatile hazardous compounds, that is, compounds that begin to off-gas or volatilize at relatively low temperatures. The quantity of gaseous material generated during the vitrification process is reduced, the thermal degradation of organic compounds in the cold top waste feed system is more uniformly controlled, the amount of volatile inorganic material released into the atmosphere is greatly reduced, and additional waste management procedures are obviated.
The processes and apparatuses of the present invention address the above-mentioned problems encountered with vitrification processes.
In accordance with the invention, a process for vitrifying a waste material containing environmentally hazardous metals or metal compounds is provided. The process includes drying the waste material using a flash dryer, preferably of the continuous loop design, where the drying is carried out at a temperature low enough to prevent substantial volatilization of hazardous metals and metal compounds contained in the waste material. The process further includes transferring the dried waste material from the flash dryer into a melting vessel, preferably utilizing either pneumatic or mechanical conveying methods, melting the waste material in the melting vessel, collecting the melted waste material, and slow, regulated cooling of the melted waste material to form vitrified finished product.
In a preferred embodiment of the invention, the flash dryer removes more than about 95% of the water from the starting waste material. The waste material may also be premixed with an additive prior to drying.
During the melting step, an electric current is established within the waste material to melt the core of the waste material, and a cold top is formed during the melting step. Exhaust gases produced during the melting step may be vented, filtered, and reintroduced into the melt unit. The resultant melted waste material is collected in an insulated mold to form waste glass by-products of various shapes and sizes.
An apparatus for vitrifying a waste material is also provided. The apparatus includes a continuous loop, direct-fired flash dryer, a metallic melting vessel, a conveyor for transferring the waste material from the flash dryer to the melting vessel, at least one port, and preferably several ports, in the melting vessel for releasing melted waste materials, and an insulated mold for collecting the melted waste materials. A preferred melting vessel has at least one port for molten glass and one port for molten metal.
In the apparatus of the invention, a means for removing gaseous materials from the dryer or melting vessel may also be included.
Other objects, details, advantages and modifications of the invention will be apparent from the following detailed description when read in conjunction with the drawings.